Arch Linux users will need to get the package from the Arch User Repository (AUR), and are looking for the "mono-pcl" package.

Setting Up

Start a new solution either by selection "Start new solution" on the welcome screen or File > New > Solution...

Select "Library" in the C# tree (Visible by default). Name it whatever you like. (We're using MyKSPProject for this tutorial. Click forward. Then Ok.)

Creating a new solution

Click Project > Edit References and follow the steps below:

Click the .NET Assembly tab and use the navigation menu on the left to browse to your KSP installation folder.

On Windows navigate to (<KSP_win>\KSP_Data\Managed) or on mac click "Browse..." and navigate to (/Users/<user>/Applications/Steam/steamapps/common/Kerbal Space Program/KSP.app/Contents/Resources/Data/Managed). Note that the KSP.app folder will show as just "KSP" (without the .app) and you can't navigate into it. Instead, navigate into there in the Finder, then right click/ctrl-click on KSP, choose "Show Package Contents", continue to the Managed directory, then drag&drop Assembly-CSharp.dll and UnityEngine.dll into the "Browse" dialog.

Select Assembly-CSharp.dll and UnityEngine.dll. More assemblies may be required depending on exactly which things your mod needs access to.

Adding References

Press Add then Ok, they should now be visible under the "References" directory of your solution.